Stated without spin — these are where a contributor's work most changes the trajectory. Seven invitations:

#	Open problem	Why it is hard	Where it stands today
1	Cross-region atomic execution	Atomicity across independent consensus zones has no trustless construction	Mitigation + economic underwriting — containment, not atomicity
2	DA at GB/s with genuinely decentralized sampling	Bandwidth and sampling honesty pull in opposite directions	Unproven at that scale, anywhere
3	Prover supply and the energy of large-scale proving	Proving is a real, heavy physical cost	An open cost, stated rather than hidden
4	Whether gravity-based placement converges	If state lands badly, cross-chain traffic never falls	A hypothesis, not yet a measurement
5	The Asset Registry as a critical trusted component	A single registry is a single point of failure	Must be rock-solid; hardening is ongoing
6	Keeping the Settlement root minimal	Every added root feature becomes a governance bottleneck	A standing constraint on every proposal
7	Relayer / watcher economics	Liveness depends on actors nobody is obliged to be	No incentive model is settled yet

Diagram

Figure 11 — Where each open problem lives — so a contributor can pick the Layer they know best.

Takeaway: XChain does not need to be perfect today; it needs to point the right way, stay honest about the path, and ship working pieces on a steady beat. These seven are open, and they are yours to take. What is not open to debate is the market reality the thesis rests on — third-party numbers, collected next.